1. Field of the Invention
The present invention relates to the treatment of frying and/or cooking oils.
2. Discussion of the Background:
All refined edible oils are made up of triglyceride molecules. Triglycerides have three fatty acids moieties attached via an ester linkage to each of three hydroxyl groups of a glycerin molecule. Between any two carbon atoms of the fatty acid moieties may be a double bond. Called points of unsaturation, double bonds are vulnerable to attack and breakdown.
Moisture from food being fried is the greatest cause of oxygen and mineral contaminants in frying oil. This moisture attacks points of unsaturation oxidatively breaking the fatty acid off from its triglyceride base. Metallic ions such as sodium, potassium and calcium react with this newly formed free fatty acid to form metallic soaps. Oil breakdown by oxygen attack also speeds the conversion of fatty acids to rancid off-flavors and off-odors.
As the concentration of soap in oil increases, these molecules begin to form water droplets called micelles that stay in the oil even at frying temperatures. Formation of micelles may be measured as a dramatic decrease in oxygen dissolved into the oil and indicates the beginning of rancidity.
The presence of soap in oil destroys the quality of a fried product. As food is immersed in contaminated oil, soap is drawn to its moisture-rich surface acting as wicks to pull oil inside the food product. Oil pickup by the food increases and the food interior becomes mushy while the exterior does not crisp.
In cooking and/or frying operations oil breakdown starts slowly and then dramatically accelerates. The slow initial conversion of fatty acids to metallic soaps is called the induction phase. This phase is slow as moisture must attack points of unsaturation in fatty acids before it passes from the oil.
The acceleration phase begins with the accumulation of metallic soaps above the minimum concentration required to support the formation of water micelles in the oil. The first off-flavors and off-odors associated with rancidity are apparent at this time.
New technology allows for the direct measurement of oxygen dissolved in edible oil for the first time. Previous tests offered only indirect measurement of oil quality with arbitrary and subjective guidelines. The results of direct measurement are independent of oil type and subjective judgment.
Oxygen content of oil declines as it breaks down into metallic soaps. Fresh oil holds oxygen in equilibrium with the atmosphere around it. As oil ages in the induction phase, oxygen content declines as moisture introduced by the food is retained. Upon the accumulation of soaps above a minimum concentration for micellular formation, the oxygen content declines dramatically.
Oxygen in these micelles attacks double bonds in the fatty acids to start oil breakdown. Anti-oxidants, which work by consuming oxygen before it can attack vulnerable fatty acids, could theoretically be used to address this problem.
Materials that function in this way naturally occur in the oil or can be added before the oil is used. However adding a greater concentration of anti-oxidant than occurs in the virgin oil can accelerate rather than delay oil breakdown. The addition of anti-oxidants after breakdown has proven ineffective.
Solid contaminants in the oil can also accelerate the induction phase of oil breakdown. Triglyceride molecules attach to the surface of a particle absorbing the heat it has collected. As each molecule is heated, a fatty acid may decompose, breaking off from the triglyceride.
Filtration slows oil breakdown by limiting the surface area available for attachment. However, oil will still breakdown in hours under the presence of moisture.
Silicates such as diatomaceous earth have been chemically altered to react with different breakdown products in the oil. Calcium silicate has been used to neutralize free fatty acids. This neutralization takes place because calcium silicate converts free fatty acids to metallic soaps. Soap makers have been adding similar materials to their processes to convert a greater percentage of oil to soap.
Another man-made material, magnesium silicate, has been used to bleach broken down oil. Color conversion does not correlate with oil performance. The application of magnesium silicate in the presence of oxygen may accelerate oil break down.
U.S. Pat. No. 3,947,602 discloses a process for increasing the useful life of cooking oil by treating the cooking oil with a food compatible acid. This treatment is reported to counteract the adverse effect of food juices dissolved in the cooking oil. The food compatible acids disclosed are citric acid, tartaric acid, acidic acid, phosphoric acid, and maleic acid.
In this process, the cooking oil is added to an aqueous solution containing the food compatible acid. U.S. Pat. No. 3,947,602's choice of acids however provides water/oil layer separation problems due to a formation of emulsions, and inadequately regenerates the oil. U.S. Pat. No. 3,947,602 also recommends using citric acid which is partially soluble in oil and consequently contaminates the regenerated oil.
U.S. Pat. No. 4,330,564, granted to one of the inventors of U.S. Pat. No. 3,947,602 proposes a solution to the problems inherent in the U.S. Pat. No. 3,947,602 process. The solution comprises immobilizing the food compatible acid on a porous rhyolite carrier. This solution however is also unsatisfactory. The use of a rhyolitic carrier requires the inconvenient use of a filter cake with filter machine to remove the rhyolite carrier supported food compatible acid from the treated oil.
Accordingly there is a strongly felt need for a process which would overcome these disadvantages and which would provide for the facile and efficient removal of undesirable impurities from cooking oil restoring the cooking oil's desirable cooking properties.